These are the modern dinosaurs, and the most biomechanically efficient living vertebrates, able to fly at 120 mph and to go around the world with amazing ease. In short, the worthy dinosaur descendants. Here we'll talk about those which range from being slightly older than ''Archaeopteryx'' to nearly as young as the modern day. On the other hand, we won't talk about historically extinct birds such as the dodo or the elephant bird: they have nothing to do with strictly prehistoric beasts, and they'll deserve a UsefulNotes Page on their own.

There are many dinosaur groups whose interrelationships are uncertain, but none are as confused as the early birds and their relatives.

For a long time, things seemed pretty simple: ''Archaeopteryx'' was a primitive bird (or, more properly, avialan). Dromaeosaurids and troodontids formed Deinonychosauria, which, along with the birds, formed Eumaniraptora ("true maniraptorans"); together with some more primitive forms, eumaniraptorans formed Paraves ("close to birds"). However, this would soon change.

In 2011, a new paravian from Late Jurassic China was named: ''Xiaotingia'' ("of Zheng Xiaoting"). The study that introduced this new theropod found a new arrangement of the paravian family tree that was quite different from the previous consensus: not only did ''Xiaotingia'' and the supposed troodontid ''Anchiornis'' end up closely related to ''Archaeopteryx'', but these forms were found to be primitive deinonychosaurs rather than birds! Predictably, popular media balleyhooed these new findings a lot. However, the authors of this study noted that support for this position was not terribly strong and was subject to change.

And change it did. For the next year or so, these three generally bounced around between being primitive birds, primitive deinonychosaurs and primitive paravians. However, in early 2013, a new phylogeny arose. This new analysis found that Deinonychosauria was not a natural group; instead, troodontids were primitive paravians, while the narrow-snouted unenlagiines and four-winged microraptorians — previously regarded as primitive dromaeosaurids — were found to be birds even more primitive than ''Archaeopteryx'', ''Anchiornis'' & ''Xiaotingia''. Unfortunately, this analysis was plagued with multiple problems (most notably, the bird group was renamed "Averaptora" ["bird thieves"], which is redundant and best ignored), so its conclusions have not received much support.

Fortunately, however, a new paravian from Late Jurassic China was announced a few short months later. Called ''Aurornis'' ("dawn bird"), the paper introducing this new form featured yet another new analysis — this one being bigger and better than any before. This one found that the bizarre scansoriopterygids, the supposed troodontid ''Eosinopteryx'', dromaeosaurids (as in most analyses, unenlagiines and microraptorians were placed in the group), troodontids, ''Aurornis'', ''Anchiornis'', ''Archaeopteryx'', ''Xiaotingia'', a new group including ''Jeholornis'' (see below) and the Madagascan ''Rahonavis'' & the supposed dromaeosaurid ''Balaur'' were progressively closer to modern birds. A later modification of the same analysis found ''Eosinopteryx'' to be closely related to ''Aurornis'' and scansoriopterygids to be closer to modern birds than ''Xiaotingia''. This analysis is tentatively accepted here, but it is unlikely that paravian relationships will be resolved anytime soon.

Scansoriopterygids were pigeon-sized animals from the latest part of the Middle Jurassic (although some thought they were as young as the Early Cretaceous). They had a body-plan apt for climbing, similar to ''Microraptor'', or rather, even more specialized; they had forelimbs longer than their hindlimbs. First discovered in 2002, they were found with feather prints around their body like many other Liaoning coelurosaurs, but their placement within the phylogenetic tree is fairly uncertain. Most analyses place them as early birds, but they could very easily be early relatives of deinonychosaurs & birds or even early oviraptorosaurs. Unluckily, only juvenile specimens are known from the few species of scansoriopterygids described, and some of their peculiar traits might just be juvenile-related and were lost in adults.

The tiny ''Epidexipteryx'' (which once contended the “smallest non-avian dinosaur” record with ''Anchiornis''), and the namesake ''Scansoripteryx'', were the only two species recognized for a long time: a third genus, "Epidendrosaurus", has been synonymized with ''Scansoriopteryx'', and there are those who still prefer that name.

''[[{{Confucius}} Confuciusornis]]'' lived in Early Cretaceous and was from the same famous Chinese Liaoning site in which the popular feathered dinosaur fossils come from. This animal had some evolved traits, for example had already lost its teeth (convergently from modern birds) and shortened its tail, but still retained an old legacy: three-clawed wings. As is easy to think from a Liaoning animal, the Confucius-bird has also preserved prints of feathers, which show two very elongated tail-feathers rather like peafowl. ''Zhongornis'', originally thought to be a transitional form between long-tailed and short-tailed birds, may be a juvenile of a bird closely related to ''Confuciusornis''. Another basal bird, ''Jeholornis'' from the same age and habitat, is also known as "Shenzhouraptor".

Another bird from Early Cretaceous China, ''[[http://en.wikipedia.org/wiki/Sapeornis Sapeornis]]'', has an interesting ScienceMarchesOn story. It was once thought to form a group (traditionally called Sapeornithidae, but properly Omnivoropterygidae) with three other birds: ''Omnivoropteryx'', ''Didactylornis'' & ''Shenshiornis''. However, further work suggests that these four birds were really just one all along; ''Sapeornis'', as the oldest of these names, is the proper one.

Yet another prehistoric Chinese bird, ''[[http://en.wikipedia.org/wiki/Gansus Gansus]]'', was described in 1984, well before the other birds mentioned in this section. This form shows similarities to various groups of living birds (and it does indeed appear to be related to them), but it is most likely a diving bird also capable of flight. It is known from much of a skeleton, but the skull is currently unknown. As with several other feathered creatures from prehistoric China, the color of its feathers has been determined; it had dark-colored plumage.

The most successful Late Cretaceous birds were the enantiornithes, whose name means "opposite birds". Why? Some skeletal features of the chest and feet are the exact opposite of those seen in modern fliers. They were a sort of middle-ways between the aforementioned Early Cretaceous birds and modern feathered guys, and were very diverse (predatory forms, aquatic forms, mud-probing forms, flightless forms, you name it). The ZergRush birds in the ''Series/WalkingWithDinosaurs'' episode about pterosaurs were enantiorns, as well as, arguably, those mentioned in the last episode which made the "omnipresent chorus" from the trees. The MirrorUniverse birds went extinct along with non-avian dinos only after the comet/asteroid.

''Hesperornis'' and ''Ichthyornis'' are two of the most famous Dinosaur Age-related birds (not counting ''Archaeopteryx''), both from Late Cretaceous North America. Since hespero is [[RuleOfCool far cooler]], here we'll mention it first. ''Hesperornis'' lived in the same habitat in which Pteranodonts, Mosasaurs, Elasmosaurs and ''Archelon''s roamed: the shallow inland sea which used to cover US Midwest at that time, dividing North America in two parallel stripes of land running from Arctic down to the south. Despite its earliness, ''Hesperornis'' was already a ''very'' derived bird. 6 ft long (the size of a human), it was flightless, with vestigial wings, short splayed legs for swimming, a long neck, and a long beak [[ToothyBird with small true teeth]]. It spent most of its life in water, but returned on land to lay its eggs. Once, the hesperorn was shown as a sort of proto-penguin with an erect pose; we know now its legs were too weak to well-substain its body, and the animal is now portrayed more similar to modern [[http://en.wikipedia/org/wiki/Grebe grebes]] and [[http://en.wikipedia/org/Loon loons]]. It was once thought it had palmated feet like a loon, but it's more likely its feet were lobed like a grebe.

The much smaller, far less striking ''Ichthyornis'' lived in the same age and habitat of the former, but this time we're coping with a sorta [[ToothyBird toothed]], long-billed proto-seagull. The ichthyorn's lifestyle was arguably similar to modern flying sea birds, catching fishes in flight or maybe diving under the sea to pursue them like modern [[http://en.wikipedia.org/wiki/Sulidae boobies]]. You can almost be certain that if pterosaurs are involved, these little guys will be depicted as pests who like to steal food from them, a behavior that while possible, has never been proven. Both birds sometimes fell preys of large marine reptiles, as shown by stomach-remains. ''Ichthyornis'' and ''Hesperornis'' were (almost) full-birdies at that point, and if alive today, they'll be taken for components of modern avifauna. In some artistic works, both ''Hesperornis'' and ''Ichthyornis'' are depicted black and white like modern gulls or penguins, but their real coloration is totally unknown.

Neornithes (meaning new birds) or colloquially "Neorns", is the name indicating the last common ancestor of all modern birds and all its descendents. Neornithes were the ''only'' Cretaceous birds which managed to overcome the mass-extinction and to make their way in the Cenozoic, the Mammal Age. It's worth noting that their descendants, our modern birdies, have much, much more species today than mammals. Most Cenozoic "new birds" were very similar to their descendants: some were rather generic-looking, while others were more specialized, but still not too different to modern avians. Furthermore, their fossil record is ''extremely'' scant, maybe the scantiest of all Vertebrates; thus, evolution of the single modern-bird lineages is mostly unknown even today, and their phylogenetic tree is full of question marks. But don't worry...there were also many exceptions to this rule: we're going to talk about these.

Long-standing paleo-fans will remember for sure the name "Diatryma": that large, flightless, large-headed predatory bird who used to hunt the small "horse" ''Eohippus'' in so many paleo-artistic depictions. Well, now poor Diatryma seems having definitively disappeared... but luckily, it's not such: it has simply changed identity. Now we have to call it ''Gastornis'' (a far less awesome name, we've to admit, but...never mind.) Whatever name should be used, this is actually one of the most enigmatic extinct birds. Recently, it was shown that it wasn't even ''carnivorous'' at all: its strong beak wasn't hooked like an eagle's, and its body frame was stocky, seemingly slow-moving. It only was an herbivore who used its bill to crack nuts, and cut vegetation. Anyway, it was a real giant in its forestal world, 40 million years ago: while mammals were still small, some birds grew to large size.

With Phorusrhacids (grassland-dwelling non-fliers), we have no doubts this time: thanks to their light weight and slender running legs, they ''were'' active hunter of small mammals. Not only that, with their strongly hooked, very eagle-like bill, they did not swallow their prey whole. It has recently been discovered they had even ''one clawed finger'' protruding from each of their tiny wings [[note]]This is not so strange as one may think: there are also living birds with this feature, the most notable being the ''two-fingered'' young [[http://en.wikipedia.org/wiki/Hoatzin Hoatzin]][[/note]], for uncertain purpose. Perhaps the most amazing-looking among all prehistoric birds, they have recently nicknamed [[CarnivoreConfusion terror-birds]] in pop- documentaries (for example, [[WalkingWithDinosaurs Prehistoric Park]].) Originary from South America, they have left a legacy in our modern world as well: the closely-related [[http://en.wikipedia.org/wiki/Seriema Seriema]] is a medium-sized South American bird whose shape and habits resemble a miniaturized "terrorbird". Even though is also nicknamed "terrorbird" sometimes, ''Gastornis''/''Diatryma'' was not related to Phorusrhacids: it has left any descendant since 40 million years. The prototypical South American ''Phorusrhacos'' (often misspelled "Phororhacos") and the North American ''Titanis'' (which first originated in South America as well) are the two stock species of the family. ----

Dromornithids were among the largest birds that ever lived (they varied in size from about as big as a cassowary to the largest and TropeNamer of the group, ''Dromornis stirtoni'', 3 meters tall and half a ton in weight); and yet, they've not gained much consideration in popular media, unlike their American contemporary counterparts, the phorusrhacids. It's probably because they likely weren't, fast, vicious killers. Instead, the [[PunnyName 'thunderbirds']], with their vast bulk, thick, robust bones, hoof-like toes and strong, crushing beaks were browsing and grazing herbivores, slowly plodding across a wetter, more wooded ancient Australian outback. Typical of Australian things, they've been given many nicknames: "thunderbirds" [[CaptainObvious obviously]] refers to their huge bulk and robust bones; "demon ducks of [[DoomyDoomsofDoom doom]]" refers to their closest living relative being the Australian magpie goose, and other waterfowl, and an old debate as to whether they were carnivores. Another addition to the list is "Mihirung", from an Aboriginal story that might mention them as the "mihirung paringmal" or Giant Emu: it is a certainty that the first people to arrive in Australia encountered them, and [[HumansAreBastards drove them to extinction]] (come on, what else wipes out species on a continent with relatively consistent climate and before the end of the ice age, eliminating climate as a variable?)

We leave (almost) definitively the flighless bird's world and start to watch more traditional fliers. Among prehistoric flying birds, among the most commonly depicted ([[RuleOfCool and most striking]]) are the teratorns, which were were very vulture-like animals. The namesake ''Teratornis'' is one of the most abundant birds in fossil record, and has been found in huge numbers in the famous Californian tar-pits in which mammalian sabretooths, giant wolves, mastodons and ground sloths have also been found. Arguably, they went to feed on the carcasses of these mammals, and remained stuck in tar just the same.

The aforementioned ''Teratornis'' had an earlier relative, which lived in South America 8 million years before: ''Argentavis'' (its name means "argentinian bird"). Why should we mention it separately? Well... simply because, along with giant pterosaurs, it deserves the GiantFlyer title more than every other prehistoric creature. Its wingspan was 25 ft, ''as much as [[StockDinosaurs Pteranodon]]''; its weight 80 kg, as much as the two-times-wider-winged ''[[StockDinosaurs Quetzalcoatlus]]''. Imagine a giant condor with a ostrich-sized body, [[Literature/ArabianNights huge roc-like wings]], a sharp uncinated beak, and a love for carrion (and maybe even an occasional hunting attitude). With no doubt, the largest flying bird ever discovered. And yet, ''Argentavis'' has yet to appear in fiction. And **heck**, it has actually had one single apparition in documentaries to date: "Paleoworld", as a ''side-note'' of Phorusrhacids! Since [[WalkingWithDinosaurs Walking With Beasts]] producers did recreate ''Argentavis'' world (the Sabretooth episode)... they wasted a perfectly good opportunity.

However, ''Argentavis'' wasn't the only GiantFlyer in the Cenozoic: we have to add the Pelagorns. These were rather albatross-like or pelican-like marine birds, but they were actually more closely related to fowl like chicken & ducks. They had two cool traits: their wingspan reached 20 ft (a bit less than ''Argentavis'') and their beak was ''toothed'', seemingly revealing the trope ToothyBird being a RealLife thing in the past. Sadly, this is not true: these "teeth" weren't real teeth, but their bill had an ondulating, pseudo-toothed edge, just like one modern bird, the duck-like [[http://en.wikipedia.org/wiki/Merganser Merganser]]. The only RealLife toothy-birds were those living alongside non-avian dinosaurs, such as ''Archaeopteryx'', ''Hesperornis'', and ''Ichthyornis'' (which weren't even closely related to any modern bird groups). Pelagorns were the new feathered version of ''Pteranodon'', almost as large as it, and went extinct only 1 million years ago. ''Osteodontornis'' is the typical member of the group.

When hesperornithines went eventually extinct at the end of the Mesozoic, a new kind of birds took soon its niche: but this time we're talking about much, ''much'' familiar-looking creatures: penguins. ''Giant'' penguins. The largest of them, ''Anthropornis'', was nearly as tall as a fully-grown human and weighed 200 kg, more than a modern ostrich; but it probably was as nice-looking as modern penguins are. Giant penguins (and penguinlike birds, such as the flightless "pelecaniform" ''Copepteryx'') swam in the southern seas for million years, until they were outcompeted 20 million years ago by a new group of large marine animals, their mammalian equivalents: seals and sea-lions.

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'''The never-never birds:''' Pterosaurs mistaken for early birds

In paleontology, it is very common for isolated bones to be misinterpreted as something very different from what they actually are. One of the best examples of this is the alleged bird ''[[http://en.wikipedia.org/wiki/Samrukia Samrukia]]'', from the Late Cretaceous of Kazakhstan. In 2011, Darren Naish of ''Blog/TetrapodZoology'' fame and his colleagues described a large jawbone, which they thought came from a large bird that they named ''Samrukia'' ("phoenix"). Later the same year, another paleontologist showed that this jaw did not come from a bird at all, but instead from a pterosaur.

History repeated itself about a year later with the reexamination of three other Cretaceous birds. In 1986, two birds were named from the Early Cretaceous of Romania. The first of these, ''[[http://en.wikipedia.org/wiki/Palaeocursornis Palaeocursornis]]'' ("ancient running bird"), was known only from a leg that showed it to be a flightless runner; the second, ''[[http://en.wikipedia.org/wiki/Eurolimnornis Eurolimnornis]]'' ("European water bird"), was a conventional flyer. In 2002, another bird was named from Late Cretaceous rocks of the United States. Called ''[[http://en.wikipedia.org/wiki/Piksi Piksi]]'' ("big bird"), it appeared to be a fowl-like ground bird. In the latest part of 2012, these birds were shown to be pterosaurs instead, and the leg of ''Palaeocursornis'' was shown to actually be an arm. Ironically, all of these forms, originally used to support the theory that Mesozoic birds were surprisingly diverse, now show that pterosaurs truly were the dominant flyers of the time.